1. Field of Invention
The present invention relates to a method of crystallizing a silicon film, more particularly, to a method of forming a polysilicon film by metal-induced crystallizing of a silicon film.
2. Discussion of the Related Art
Field effect mobility and stability of light irradiation of a polysilicon thin film transistor (hereinafter abbreviated TFT) are excellent when compared to those of an amorphous silicon TFT. Therefore, polysilicon TFTs are used as driving devices in active matrix liquid crystal displays (AMLCDs) as well as basic devices in peripheral circuits.
There are various methods of fabricating polysilicon and these methods are mainly divided into two classes: methods of depositing polysilicon directly and methods of depositing amorphous silicon and subsequently crystallizing it.
The methods of depositing polysilicon directly include chemical vapor deposition (CVD), photo-CVD, HR (hydrogen radical) CVD, ECR (electron cyclotron resonance) CVD, PE (plasma enhanced) CVD, LP (low pressure) CVD, and the like.
Methods of depositing amorphous silicon and subsequently crystallizing it include solid phase crystallization (hereinafter abbreviated SPC), liquid phase crystallization (LPC), crystallization by a pulse laser such as an excimer laser, and the like.
A recently introduced method for crystallizing amorphous silicon into the polycrystalline form lowers the temperature of crystallization to fabricate a TFT-LCD of large size. It is known as Metal Induced Crystallization (hereinafter abbreviated MIC). In MIC, the temperature of silicon crystallization is reduced to below 500xc2x0 C. by contacting specific metals with amorphous silicon. The MIC effect is verified with various kinds of metals.
Crystallization by MIC is caused by various factors, depending on the kinds of metals present. Namely, the crystallization depends on the species of metals contacted with silicon.
Effects of metals such as Al, Au, Ag, and the like are ruled by diffusion of Si at the interface between the metal and the Si. The silicide formed at the interface is in a pseudo-stable state, hence the silicide lowers the crystallization energy, thereby accelerating the silicon crystallization.
Conversely, effects of other metals such as Ni, Ti, and the like are ruled by diffusion of the metal during annealing. Namely, the silicide state is formed at the interface between the metal and silicon by the diffusion of the metal into the silicon. The silicide thus lowers the temperature of crystallization. In MIC, using nickel as the metal, NiSi2, the last phase of the Ni silicide, nucleates and accelerates the Si crystallization.
FIG. 1A and FIG. 1B show cross-sectional views of crystallizing a silicon film according to the related art method.
Referring to FIG. 1A, a silicon oxide buffer layer 10 is formed on an insulating substrate 100. An amorphous silicon film 11 is deposited on the buffer layer 10. Then, a metal film, used as a crystallization catalyst, such as an Ni film 13, is formed on the amorphous silicon film 11.
The Ni film 13 is deposited as a thin film on the amorphous silicon film 11 by a conventional method of sputtering.
Referring to FIG. 1B, the above insulating substrate 100 is heat-treated to crystallize the amorphous silicon film 11. As a result of the heat treatment, a silicide phase (not shown) is formed by Ni diffusion into the silicon layer. Then, the amorphous silicon film 11 is crystallized into a polysilicon film 19 at a lowered temperature since the silicide accelerates the crystallization of the silicon film.
The drawback of this related method is that the silicon crystallization is carried out by forming a Ni film 13 which has a predetermined thickness and that is used as a crystallization catalyst. Herein, nickel materials remains in the crystallized silicon film more or less, yielding TFTs of polysilicon film contaminated massively by the Ni, having poor device characteristics and that are improper for devices.
Accordingly, embodiments of the present invention provide a method of crystallizing a silicon film that substantially obviates one or more of the problems due to the limitations and disadvantages of the related art.
An object of the present invention is to provide a method of crystallizing a silicon film wherein an amorphous silicon film is crystallized by forming a silicon layer containing a catalytic substance for silicon crystallization as well as by using MIC.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as in the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the method for crystallizing a silicon film includes the steps of forming a silicon layer containing a catalyst for silicon crystallization, and crystallizing the silicon layer. The method further includes the steps of forming an amorphous silicon layer between the substrate and the silicon layer, and crystallizing the silicon layer and the amorphous silicon layer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.